Fluid pumps such as a sump pump 1 shown in FIG. 1 consist essentially of a housing 2, which houses a pumping cavity 2c, a freely rotatable driving shaft 3 located within the pumping cavity and an impeller 4 fixed to one end of the driving shaft.
The housing 2 is provided with an intake opening 2a positioned directly in line with the driving shaft 3 and an discharge opening 2b positioned approximately tangentially to the disc shaped impeller 4.
The cross sectional area of the discharge opening 2b gradually increases towards the exit opening to serve the function of a diffuser.
Also the impeller 4 is provided with a base section 4a which serves as the connecting section with the driving shaft 3, and a number of vanes 5 extending approximately radially from the base section 4a. The vanes are not connected to the shaft, as shown in FIG. 2, and is known as the open-type vanes.
This type of sump pump operates by the action of the rotatable shaft 3 which moves the impeller 4 to cause the water contained inside the housing 2 to be moved from the base section 4a and to be hurled against the housing walls, by the centrifugal force. Continued rotation causes an increase in the water pressure, leading to discharging of the water from the discharge opening 2b and, at the same time, lifting of additional volume of water into the intake opening 2a.
Such a design of the impeller leads to lowering of the pumping efficiency because the pressured water contained in a vane section between the adjacent vanes 5 tends to leak in the direction of 4b into the adjacent section through the small clearance between the vanes 5 and the inside of the pumping chamber, shown in FIG. 1. It should be remembered that this clearance itself is, indeed, a part of the fluid passage routes in the conventional open-type vane design and it cannot be eliminated.
In order to solve such problems, an impeller 6 of a design shown in FIG. 3 and another impeller 7 of a design shown in FIG. 4 have been proposed.
The impeller 6 shown in FIG. 3 is a semi-open type and has a disc-shaped solid base section 8 which extends beyond the vanes so as to contain the water more effectively inside the vane section. The objective is to prevent the leaking of water in between the vane sections.
The impeller 7 shown in FIG. 4 is a closed type and has an additional closure in the form of a shroud covering 9 over the vanes 5 so as to seal in the water inside the vane sections. The same objective as in the previous design is retained, and that is to prevent the leaking of water between the vane sections.
However, the presence of the extended base 8 and the shroud cover 9, results in the creation of a stagnant water region between the vanes and the impeller housing 2, and the water in this region then effectively becomes isolated from the rotational action of the impeller.
Accordingly, the hydrostatic pressure on one side of the impeller increases, resulting in the development of a thrust loading on the impeller 6, which obstructs smooth rotation of said impeller 6.
The resulting viscous drag between the housing 2 and the base 8 increases also and affects the pumping efficiency of the impeller 6.
In the case of the impeller 7, since there are effectively two protective shrouds, 8 and 9, the thrust loading on the impeller 7 is decreased. However, because of an increase in the opposing surface areas between the impeller surface and the interior surface area of the pump housing, there is a corresponding increase in the viscous drag, which creates one reason for the loss of pumping efficiency.
Furthermore, the impeller 7 is prone to creating a pressure differential between the inside and outside of the intake opening 7a, and this pressure differential creates a phenomenon of reverse flow of the fluid from inside the pumping chamber 2c.
Returning to FIG. 1, the water driven by the centrifugal force travels in the tangential direction along the inside surface of the housing 2, and is discharged from the discharge opening 2b.
To improve the pumping efficiency of the impeller 1, the following improvements to said impeller are being sought.
That is, it can be recognized that there are two energy conservation requirements, which are represented by the ratio of pressure energy and the residual kinetic energy of the pressurized water, at the discharge opening 2b. On the one hand, it is desirable to maintain the water pressure energy right up to the discharge opening, and on the other, to convert as much of the kinetic energy of the moving water into pressure energy, except those energy components which can increase the average exit velocity of the discharging water.
This is explained in reference to data presented in FIG. 6 which shows the relationship between the pressure and the flow volume. The static pressure P.sub.0 at the exit region of the impeller 4 is relatively insensitive to the flow volume. On the other hand, theoretical calculations demonstrate that both the static pressure P.sub.1 ', at the entry region to the discharge opening 2b, and the exit pressure P.sub.2, at the final exit opening, decrease with flow volume. In the conventional sump pump 1, the pressure P.sub.1 decreases even more rapidly with the flow volume than the calculations indicate.
The reason for this phenomenon is considered to be the following.
The main body of the fluid flowing into the discharge opening 2b is generally flowing tangentially to the inner surface of the housing 2, as illustrated in FIG. 5, however, the remaining portion of the fluid which flows close to the inner surface of the housing 2 experiences cavitation when it encounters a protrusion A disposed on the inner portion of the discharge opening 2b of the housing 2. Thus, there is an effective narrowing of the opening area of the entry region of the discharge opening 2b, which leads to an increase in the fluid velocity at this location.